ASK1 inhibits astroglial development via p38 mitogen-activated protein kinase and promotes neuronal differentiation in adult hippocampus-derived progenitor cells

Anti-inflammatory effects of TNF-α and ASK1 inhibitory compounds isolated from Schkuhria pinnata used for the treatment of dermatitis

ETHNO-PHARMACOLOGICAL RELEVANCE

The Andean Schkuhria pinnata species commonly known as 'Canchalagua' is used as an infusion in Andean countries to treat various anti-inflammatory and skin-related pathologies.

AIM OF THE STUDY

This study determined the anti-inflammatory activity of the aqueous extract from Schkuhria pinnata, identified compounds with high biological activity and performed a structure-activity relationship analysis to determine their binding mechanism.

MATERIALS AND METHODS

A bio-guided isolation of the active compounds of Schkuhria pinnata was carried out by selecting the most active sub-extracts and fractions to test their anti-inflammatory activity against the ASK1 and TNF-α cytokines.

RESULTS

Three compounds were obtained, and their structures were elucidated by nuclear magnetic resonance. The compounds were (3R,4R)-4-(3,4-dimethoxybenzyl)-3-(4-hydroxy-3-methoxybenzyl) dihydrofuran-2(3H)-one (1), N-[2,3-dihydro-1,3-dimethyl-6-[(2R)-2-methyl-1-piperazinyl]-2-oxo-1H-benzimidazol-5-yl]-2-methoxybenzamide (2), and N-hydroxy-1-cyclopentene-1-carboxamide (3). Regarding their anti-inflammatory activity, the three compounds inhibited the TNF-α and ASK1 cytokines, however, compound 2 was the most active, with an IC50 of 19.08 and 8.94 nM, respectively.

CONCLUSION

The anti-inflammatory activity of the aqueous extract of Schkuhria pinnata was evaluated, followed by the isolation of three compounds and the study of their pharmacological activity. The three compounds have been shown as promising treatment against dermatitis, confirming at the same time their traditional use.

Adult neurogenesis: a real hope or a delusion?

Adult neurogenesis, the process of creating new neurons, involves the coordinated division, migration, and differentiation of neural stem cells. This process is restricted to neurogenic niches located in two distinct areas of the brain: the subgranular zone of the dentate gyrus of the hippocampus and the subventricular zone of the lateral ventricle, where new neurons are generated and then migrate to the olfactory bulb. Neurogenesis has been thought to occur only during the embryonic and early postnatal stages and to decline with age due to a continuous depletion of neural stem cells. Interestingly, recent years have seen tremendous progress in our understanding of adult brain neurogenesis, bridging the knowledge gap between embryonic and adult neurogenesis. Here, we discuss the current status of adult brain neurogenesis in light of what we know about neural stem cells. In this notion, we talk about the importance of intracellular signaling molecules in mobilizing endogenous neural stem cell proliferation. Based on the current understanding, we can declare that these molecules play a role in targeting neurogenesis in the mature brain. However, to achieve this goal, we need to avoid the undesired proliferation of neural stem cells by controlling the necessary checkpoints, which can lead to tumorigenesis and prove to be a curse instead of a blessing or hope.

JUN upregulation drives aberrant transposable element mobilization, associated innate immune response, and impaired neurogenesis in Alzheimer's disease

Adult neurogenic decline, inflammation, and neurodegeneration are phenotypic hallmarks of Alzheimer's disease (AD). Mobilization of transposable elements (TEs) in heterochromatic regions was recently reported in AD, but the underlying mechanisms are still underappreciated. Combining functional genomics with the differentiation of familial and sporadic AD patient derived-iPSCs into hippocampal progenitors, CA3 neurons, and cerebral organoids, we found that the upregulation of the AP-1 subunit, c-Jun, triggers decondensation of genomic regions containing TEs. This leads to the cytoplasmic accumulation of HERVK-derived RNA-DNA hybrids, the activation of the cGAS-STING cascade, and increased levels of cleaved caspase-3, suggesting the initiation of programmed cell death in AD progenitors and neurons. Notably, inhibiting c-Jun effectively blocks all these downstream molecular processes and rescues neuronal death and the impaired neurogenesis phenotype in AD progenitors. Our findings open new avenues for identifying therapeutic strategies and biomarkers to counteract disease progression and diagnose AD in the early, pre-symptomatic stages.

Involvement of p38 in Age-Related Decline in Adult Neurogenesis via Modulation of Wnt Signaling

Neurogenesis in specific brain regions in adult mammals decreases with age. Progressive reduction in the proliferation of neural stem and progenitor cells (NS/PCs) is a primary cause of this age-associated decline. However, the mechanism responsible for this reduction is poorly understood. We identify p38 MAPK as a key factor in the proliferation of neural progenitor cells (NPCs) in adult neurogenic niches. p38 expression in adult NS/PCs is downregulated during aging. Deletion of p38α in NS/PCs specifically reduces the proliferation of NPCs but not stem cells. Conversely, forced expression of p38α in NS/PCs in the aged mouse subventricular zone (SVZ) restores NPC proliferation and neurogenesis, and prevents age-dependent SVZ atrophy. We also found that p38 is necessary for suppressing the expression of Wnt antagonists DKK1 and SFRP3, which inhibit the proliferation of NPCs. Age-related reduction in p38 thus leads to decreased adult neurogenesis via downregulation of Wnt signaling.

Regulation of Redox Homeostasis by Nonthermal Biocompatible Plasma Discharge in Stem Cell Differentiation

Recently, a growing body of evidence has shown the role of reactive species as secondary messengers in cell proliferation and differentiation, as opposed to the harmful metabolism byproducts that they were previously solely recognized as. Thus, the balance of intracellular reduction-oxidation (redox) homeostasis plays a vital role in the regulation of stem cell self-renewal and differentiation. Nonthermal biocompatible plasma (NBP) has emerged as a novel tool in biomedical applications. Recently, NBP has also emerged as a powerful tool in the tissue engineering field for the surface modification of biomaterial and the promotion of stem cell differentiation by the regulation of intracellular redox biology. NBP can generate various kinds of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which may play the role of the second passenger in the cell signaling network and active antioxidant system in cells. Herein, we review the current knowledge on mechanisms by which NBP regulates cell proliferation and differentiation through redox modification. Considering the importance of redox homeostasis in the regulation of stem cell differentiation, understanding the underlying molecular mechanisms involved will provide important new insights into NBP-induced stem cell differentiation for tissue engineering.

Apoptosis signal-regulating kinase 1 mediates the inhibitory effect of hepatocyte nuclear factor-4α on hepatocellular carcinoma

Previous studies provided substantial evidence of a striking suppressive effect of hepatocyte nuclear factor 4α (HNF4α) on hepatocellular carcinoma (HCC). Apoptosis signal-regulating kinase 1 (ASK1) is involved in death receptor-mediated apoptosis and may acts as a tumor suppressor in hepatocarcinogenesis. However, the status and function of ASK1 during HCC progression are unclear. In this study, we found that HNF4α increased ASK1 expression by directly binding to its promoter. ASK1 expression was dramatically suppressed and correlated with HNF4α levels in HCC tissues. Reduced ASK1 expression was associated with aggressive tumors and poor prognosis for human HCC. Moreover, ASK1 inhibited the malignant phenotype of HCC cells in vitro. Intratumoral ASK1 injection significantly suppressed the growth of subcutaneous HCC xenografts in nude mice. More interestingly, systemic ASK1 delivery strikingly inhibited the growth of orthotopic HCC nodules in NOD/SCID mice. In addition, inhibition of endogenous ASK1 partially reversed the suppressive effects of HNF4α on HCC. Collectively, this study highlights the suppressive effect of ASK1 on HCC and its biological significance in HCC development. These outcomes broaden the knowledge of ASK1 function in HCC progression, and provide a novel potential prognostic biomarker and therapeutic target for advanced HCC.

Stem cells, mitochondria and aging

Decline in metabolism and regenerative potential of tissues are common characteristics of aging. Regeneration is maintained by somatic stem cells (SSCs), which require tightly controlled energy metabolism and genomic integrity for their homeostasis. Recent data indicate that mitochondrial dysfunction may compromise this homeostasis, and thereby contribute to tissue degeneration and aging. Progeroid Mutator mouse, accumulating random mtDNA point mutations in their SSCs, showed disturbed SSC homeostasis, emphasizing the importance of mtDNA integrity for stem cells. The mechanism involved changes in cellular redox-environment, including subtle increase in reactive oxygen species (H₂O₂and superoxide anion), which did not cause oxidative damage, but disrupted SSC function. Mitochondrial metabolism appears therefore to be an important regulator of SSC fate determination, and defects in it in SSCs may underlie premature aging. Here we review the current knowledge of mitochondrial contribution to SSC dysfunction and aging. This article is part of a Special Issue entitled: Mitochondrial Dysfunction in Aging.

Development of an HTS-compatible assay for the discovery of ASK1 signalosome inhibitors using alphascreen technology

Genetic target validation studies have demonstrated that the apoptosis signal-regulating kinase 1 (ASK1) represents an important target for the treatment of rheumatoid arthritis, cardiac diseases, and several neurodegenerative disorders. To identify small-molecule inhibitors of ASK1, we have developed a high-throughput screening-compatible, homogenous, biochemical assay using AlphaScreen technology. This novel assay design utilizes purified stress-activated ASK1 signalosome complex, and it monitors phosphorylation of its full-length native substrate, MKK6. The assay has been optimized in a 384-well format and validated by screening the Sigma LOPAC library. The results presented here demonstrate that the assay is sensitive and robust with a Z' factor value of 0.88±0.04 and a signal-to-background ratio of 11, indicating that this assay can be used to screen large chemical libraries to discover novel inhibitors of ASK1.

Apoptosis signal-regulating kinase 1 (ASK1) is linked to neural stem cell differentiation after ischemic brain injury

Neural stem cells (NSCs) have been suggested as a groundbreaking solution for stroke patients because they have the potential for self-renewal and differentiation into neurons. The differentiation of NSCs into neurons is integral for increasing the therapeutic efficiency of NSCs during inflammation. Apoptosis signal-regulating kinase 1 (ASK1) is preferentially activated by oxidative stress and inflammation, which is the fundamental pathology of brain damage in stroke. ASK1 may be involved in the early inflammation response after stroke and may be related to the differentiation of NSCs because of the relationship between ASK1 and the p38 mitogen-activated protein kinase pathway. Therefore, we investigated whether ASK1 is linked to the differentiation of NSCs under the context of inflammation. On the basis of the results of a microarray analysis, we performed the following experiments: western blot analysis to confirm ASK1, DCX, MAP2, phospho-p38 expression; fluorescence-activated cell sorting assay to estimate cell death; and immunocytochemistry to visualize and confirm the differentiation of cells in brain tissue. Neurosphere size and cell survival were highly maintained in ASK1-suppressed, lipopolysaccharide (LPS)-treated brains compared with only LPS-treated brains. The number of positive cells for MAP2, a neuronal marker, was lower in the ASK1-suppressed group than in the control group. According to our microarray data, phospho-p38 expression was inversely linked to ASK1 suppression, and our immunohistochemistry data showed that slight upregulation of ASK1 by LPS promoted the differentiation of endogenous, neuronal stem cells into neurons, but highly increased ASK1 levels after cerebral ischemic damage led to high levels of cell death. We conclude that ASK1 is regulated in response to the early inflammation phase and regulates the differentiation of NSCs after inflammatory-inducing events, such as ischemic stroke.

Skeletal cell differentiation is enhanced by atmospheric dielectric barrier discharge plasma treatment

Enhancing chondrogenic and osteogenic differentiation is of paramount importance in providing effective regenerative therapies and improving the rate of fracture healing. This study investigated the potential of non-thermal atmospheric dielectric barrier discharge plasma (NT-plasma) to enhance chondrocyte and osteoblast proliferation and differentiation. Although the exact mechanism by which NT-plasma interacts with cells is undefined, it is known that during treatment the atmosphere is ionized generating extracellular reactive oxygen and nitrogen species (ROS and RNS) and an electric field. Appropriate NT-plasma conditions were determined using lactate-dehydrogenase release, flow cytometric live/dead assay, flow cytometric cell cycle analysis, and Western blots to evaluate DNA damage and mitochondrial integrity. We observed that specific NT-plasma conditions were required to prevent cell death, and that loss of pre-osteoblastic cell viability was dependent on intracellular ROS and RNS production. To further investigate the involvement of intracellular ROS, fluorescent intracellular dyes Mitosox (superoxide) and dihydrorhodamine (peroxide) were used to assess onset and duration after NT-plasma treatment. Both intracellular superoxide and peroxide were found to increase immediately post NT-plasma treatment. These increases were sustained for one hour but returned to control levels by 24 hr. Using the same treatment conditions, osteogenic differentiation by NT-plasma was assessed and compared to peroxide or osteogenic media containing β-glycerolphosphate. Although both NT-plasma and peroxide induced differentiation-specific gene expression, neither was as effective as the osteogenic media. However, treatment of cells with NT-plasma after 24 hr in osteogenic or chondrogenic media significantly enhanced differentiation as compared to differentiation media alone. The results of this study show that NT-plasma can selectively initiate and amplify ROS signaling to enhance differentiation, and suggest this technology could be used to enhance bone fusion and improve healing after skeletal injury.

TGF-β-activated kinase 1 (TAK1) and apoptosis signal-regulating kinase 1 (ASK1) interact with the promyogenic receptor Cdo to promote myogenic differentiation via activation of p38MAPK pathway

p38MAPK plays an essential role in the transition of myoblasts to differentiated myotubes through the activation of MyoD family transcription factors. A promyogenic cell surface molecule, Cdo, promotes myogenic differentiation mainly through activation of the p38MAPK pathway. Two MAP3Ks, TAK1 and ASK1, can activate p38MAPK via MKK6 in various cell systems. Moreover TAK1 has been shown to promote myogenic differentiation via p38MAPK activation. In this study, we hypothesized that TAK1 and ASK1 might function as MAP3Ks in Cdo-mediated p38MAPK activation during myoblast differentiation. Both ASK1 and TAK1 were expressed in myoblasts and interacted with the cytoplasmic tail of Cdo and a scaffold protein, JLP. The depletion of TAK1 or ASK1 in C2C12 cells decreased myoblast differentiation, whereas overexpression of TAK1 or ASK1 in C2C12 cells enhanced myotube formation. In agreement with this, overexpression of ASK1 or TAK1 resulted in enhanced p38MAPK activation, and their knockdown inhibited p38MAPK in C2C12 cells. Overexpression of TAK1 or ASK1 in Cdo(-/-) myoblasts and Cdo-depleted C2C12 cells restored p38MAPK activation as well as myotube formation. Furthermore, ASK1 and TAK1 compensated for each other in p38MAPK activation and myoblast differentiation. Taken together, these findings suggest that ASK1 and TAK1 function as MAP3Ks in Cdo-mediated p38MAPK activation to promote myogenic differentiation.

Peroxiredoxin-2 protects against 6-hydroxydopamine-induced dopaminergic neurodegeneration via attenuation of the apoptosis signal-regulating kinase (ASK1) signaling cascade

The peroxiredoxin (PRX) family of antioxidant enzymes helps maintain the intracellular reducing milieu and suppresses apoptosis in non-neuronal cells. However, whether PRX can inhibit neuronal apoptosis through specific signaling mechanisms remains poorly understood. Induction of PRX2, the most abundant neuronal PRX, occurs in Parkinson's disease (PD) patient brains, but its functional impact is unclear. In the present study, we used the dopaminergic (DA) toxin 6-hydroxydopamine (6-OHDA) to model PD and explore the protective effect and mechanisms of PRX on DA neurons. Of the 2-cysteine PRXs that were tested in MN9D DA neurons, endogenous PRX2 was most beneficial to cell survival. Lentivirus-mediated PRX2 overexpression conferred marked in vitro and in vivo neuroprotection against 6-OHDA toxicity in DA neurons, and preserved motor functions involving the dopamine system in mouse. In addition to its role as an antioxidant enzyme, PRX2 exhibited anti-apoptotic effects in DA neurons via suppression of apoptosis signal-regulating kinase (ASK1)-dependent activation of the c-Jun N-terminal kinase/c-Jun and p38 pro-death pathways, which are also activated in DA neurons of postmortem PD brains. PRX2 inhibited 6-OHDA-induced ASK1 activation by modulating the redox status of the endogenous ASK1 inhibitor thioredoxin (Trx). PRX2 overexpression maintained Trx in a reduced state by inhibiting the cysteine thiol-disulfide exchange, thereby preventing its dissociation from ASK1. This study describes a previously undefined mechanism by which redox-sensitive molecules signal via apoptotic pathways in response to PD-relevant toxic stress in DA neurons. Our results also suggest that PRX2 and ASK1 may be potential targets for neuroprotective intervention in PD.

TLX activates MASH1 for induction of neuronal lineage commitment of adult hippocampal neuroprogenitors

The orphan nuclear receptor TLX has been proposed to act as a repressor of cell cycle inhibitors to maintain the neural stem cells in an undifferentiated state, and prevents commitment into astrocyte lineages. However, little is known about the mechanism of TLX in neuronal lineage commitment and differentiation. A majority of adult rat hippocampus-derived progenitors (AHPs) cultured in the presence of FGF express a high level of TLX and a fraction of these cells also express the proneural gene MASH1. Upon FGF withdrawal, TLX rapidly decreased, while MASH1 was intensely expressed within 1h, decreasing gradually to disappear at 24h. Adenoviral transduction of TLX in AHP cells in the absence of FGF transiently increased cell proliferation, however, later resulted in neuronal differentiation by inducing MASH1, Neurogenin1, DCX, and MAP2ab. Furthermore, TLX directly targets and activates the MASH1 promoter through interaction with Sp1, recruiting co-activators whereas dismissing the co-repressor HDAC4. Conversely, silencing of TLX in AHPs decreased beta-III tubulin and DCX expression and promoted glial differentiation. Our results thus suggest that TLX not only acts as a repressor of cell cycle and glial differentiation but also activates neuronal lineage commitment in AHPs.

Directed neural lineage differentiation of adult hippocampal progenitor cells via modulation of hippocampal cholinergic neurostimulating peptide precursor expression

Hippocampal cholinergic neurostimulating peptide (HCNP), originally purified from young rat hippocampus, has been known to promote the differentiation of septo-hippocampal cholinergic neurons. Recently, the precursor protein of HCNP (HCNP-pp) has also received attention as a multifunctional protein with roles, in addition to serving as the HCNP precursor, such as acting as an ATP-binding protein, a Raf kinase inhibitor protein (RKIP), and phosphatidylethanolamine-binding protein (PEBP). In particular, the function of RKIP has attracted attention over several years for its role in controlling cellular proliferation and metastasis in cancer cells. HCNP-pp is also thought to be important in regulating the proliferation and differentiation of neuronal cells in vitro and in vivo by modification of the MAPK cascade. In the present study, we used cultured adult rat hippocampal progenitor cells (AHPs), which are thought to be important for memory formation, and focused on the role of HCNP-pp in adult neurogenesis, namely, the production of new neurons from neural stem/progenitor cells. We found that HCNP-pp expression in AHPs was closely associated with differentiation into MAP2ab-positive neurons and RIP-positive oligodendrocytes, but not into GFAP-positive astrocytes. By contrast, a down-regulated HCNP-pp expression in AHPs accompanied differentiation into GFAP-positive astrocytes. Direct manipulations of HCNP-pp via viral over-expression or siRNA downregulation further confirmed the HCNP-pp contribution to specific neural lineage commitment of AHPs. Our results show that the expression level of HCNP-pp acts as a key regulator for differentiation of cultured AHPs into specific neural lineages, indicating that the control of neural stem cell fate can be achieved via the HCNP-pp pathway.

Mu and kappa opioids modulate mouse embryonic stem cell-derived neural progenitor differentiation via MAP kinases

As embryonic stem cell-derived neural progenitors (NPs) have the potential to be used in cell replacement therapy, an understanding of the signaling mechanisms that regulate their terminal differentiation is imperative. In previous studies, we discovered the presence of functional mu opioid receptors (MOR) and kappa opioid receptors (KOR) in mouse embryonic stem cells and NPs. Here, MOR and KOR immunoreactivity was detected in NP-derived oligodendrocytes during three stages of their maturation in vitro. Moreover, we examined the modulation of retinoic acid-induced NP differentiation to astrocytes and neurons by mu, [D-ala(2), mephe(4), gly-ol(5)] enkephalin, or kappa, U69, 593, opioids. Both opioid agonists inhibited NP-derived neurogenesis and astrogenesis via their corresponding receptors as determined by immunocytochemistry. By administering selective inhibitors, we found that opioid inhibition of NP-derived astrogenesis was driven via extracellular-signal regulated kinase (ERK), while the p38 mitogen-activated protein kinase pathway was implicated in opioid attenuation of neurogenesis. In addition, mu and kappa opioids stimulated oligodendrogenesis from NP-derived NG2(+) oligodendrocyte progenitors via both ERK and p38 signaling pathways. Accordingly, both opioids induced ERK phosphorylation in NG2(+) cells. These results indicate that small molecules, such as MOR and KOR agonists may play a modulatory role in NP terminal differentiation.

Cdo promotes neuronal differentiation via activation of the p38 mitogen-activated protein kinase pathway

Neural basic helix-loop-helix transcription factors (bHLHs) control many aspects of neurogenesis, such as proliferation, fate determination, and differentiation. We have previously shown that the promyogenic cell surface receptor Cdo modulates the Cdc42 and p38 mitogen-activated protein kinase (MAPK) pathways via a direct association with two scaffold-type proteins, JLP and Bnip-2, to regulate activities of myogenic bHLH factors and myogenic differentiation. We report here that Cdo uses similar regulatory mechanisms to promote neuronal differentiation. Expression of JLP, a scaffold protein for p38MAPK, and Bnip-2, a regulator of Cdc42, is increased during differentiation of C17.2 neural precursor cells and P19 embryonal carcinoma cells. These molecules regulate Cdc42 and p38MAPK activities, which increase in a Cdo-dependent manner during neuronal differentiation of C17.2 cells and retinoic acid-treated P19 cells. Furthermore, enhancement or reduction of Cdc42 and p38MAPK activities enhances or reduces, respectively, neuronal differentiation of these cell lines. Cdc42 and p38MAPK activities also promote heterodimerization of neurogenin1 and E47, suggesting that one way they promote neurogenesis is via regulation of neural bHLH factor activities. These results imply that a conserved intracellular signaling mechanism initiated by Cdo regulates the activities of tissue-specific bHLH factors and therefore functions as a key regulator of differentiation of several different cell lineages.

Inhibitors of p38 mitogen-activated protein kinase enhance proliferation of mouse neural stem cells

The p38 mitogen-activated protein kinase (MAPK) is induced in response to environmental stress. Although p38 MAPK has been implicated in diverse cellular processes, including cell proliferation, differentiation, and survival of differentiated cells in the central nervous system (CNS), the expression profile and roles of p38 MAPK in the developing brain remain largely unknown. In the present study, we demonstrate that p38 MAPK is expressed predominantly in nestin-positive cells in the cerebral cortex in embryonic day 10 (E10) brain and that expression of the protein decreases gradually during development. To investigate the roles of p38 MAPK in the embryonic brain, two selective p38 MAPK inhibitors, SB202190 and SB203580, were added to the primary neuronal cultures from E10-E14 brains. After 7 days of exposure to these inhibitors, but not SB202474, a negative analog of SB203580, numerous large neurospheres were present. MAPK inhibitors also selectively increased the growth rate of neural stem cells (NSCs) purified from secondary neurospheres and the number of bromodeoxyuridine-positive NSCs. Thus, p38 MAPK inhibitors are potent stimulators of NSC proliferation, and p38 MAPK may be an intrinsic negative regulator of NSC proliferation during early brain development.

Opposing effects of retinoid signaling on astrogliogenesis in embryonic day 13 and 17 cortical progenitor cells

All-trans retinoic acid (RA) is a differentiation factor in many tissues. However, its role in astrogliogenesis has not been extensively studied. Here, we investigated the effect of RA on the regulation of astrogliogenesis at different cortical developmental stages. We prepared rat cortical progenitor cells from embryonic day (E) 13 and E17, which correspond to the beginning of neurogenic and astrogliogenic periods, respectively. Surprisingly, RA promoted astrogliogenesis at E17 but inhibited astrogliogenesis induced by ciliary neurotrophic factor (CNTF) at E13. The inhibitory effect of RA on astrogliogenesis at E13 was not due to premature commitment of progenitors to a neuronal or oligodendroglial lineage. Rather, RA retained more progenitors in a proliferative state. Furthermore, RA inhibition of astrogliogenesis at E13 was independent of STAT3 signaling and required the function of the alpha and beta isoforms of the RA receptors (RAR). Moreover, the differential response of E13 and E17 progenitors to RA was due to differences in the intrinsic properties of these cells that are preserved in vitro. The inhibitory effect of RA on cytokine-induced astrogliogenesis at E13 may contribute to silencing of any potential precocious astrogliogenesis during the neurogenic period.

Stable expression of intracellular Notch suppresses v-Src-induced transformation in avian neural cells

Understanding how disruption of differentiation contributes to the cancer cell phenotype is required to identify alterations essential for malignant transformation and provide experimental basis for their correction. We investigated whether primary quail neuroretina cells, transformed by a conditional v-Src mutant (QNR/v-src(ts)), could revert to a normal phenotype, in response to the stable expression of constitutively active Notch1 intracellular domain (ICN). This model system was chosen because Notch signaling plays an instructive role in cell fate determination during NR development, and because the intrinsic capacity of QNR cultures to differentiate is blocked by v-Src. We report that stable ICN expression results in suppression of QNR/v-src(ts) cell transformation in the presence of an active oncoprotein. This phenotypic reversion coincides with a major switch in cell identity, as these undifferentiated cells acquire glial differentiation traits. Both changes appear to be mediated by CBF, a transcription factor that binds to ICN and activates target genes. Cells restored to a normal and differentiated phenotype have undergone changes in the functioning of signaling effectors, essentially regulating cell morphology and cytoskeleton organization. This dominant interference may be partially mediated by an autocrine/paracrine mechanism, as revertant cells secrete a factor(s), which inhibits transformation properties of QNR/v-src(ts) cells.

GRP78-binding protein regulates cAMP-induced glial fibrillary acidic protein expression in rat C6 glioblastoma cells

We previously reported that a novel GRP78-binding protein (GBP) is predominantly expressed in rat brain and its expression declines through the aging process. To characterize its biological function, we established C6 glioblastoma cells that stably overexpressed GBP. Stable overexpression of GBP attenuated cAMP-induced expression of the glial fibrillary acidic protein (GFAP) gene, which was accompanied by a decrease in cAMP-induced signal transducer and activators of transcription 3 (STAT3) phosphorylation. Other distinct cAMP-induced events, including a transient reduction in extracellular signal-regulated protein kinase phosphorylation and a slowdown in cell proliferation, were hardly affected by GBP overexpression. Most importantly, treatment with siRNA against endogenous GBP markedly downregulated GBP expression in C6 glioblastoma cells, and dramatically augmented cAMP-induced GFAP mRNA expression in parallel with hyper-phosphorylation of STAT3. These results suggest a novel function of GBP in regulating GFAP gene expression via STAT3 phosphorylation.

ASK-1 (apoptosis signal-regulating kinase 1) is a direct E2F target gene

In the present study, we show that E2Fs (E2 promoter-binding factors) regulate the expression of ASK-1 (apoptosis signal-regulating kinase 1), which encodes a mitogen-activated protein kinase kinase kinase, also known as MAP3K5. Its mRNA expression is cell-cycle-regulated in human T98G cells released from serum starvation. Moreover, overexpression and RNA interference experiments support the requirement of endogenous E2F/DP (E2F dimerization partner) activity for ASK-1 expression. Characterization of the human ASK-1 promoter demonstrates that the -95/+11 region is critical for E2F-mediated up-regulation. Chromatin immunoprecipitation assays show that E2F1-E2F4 are bound in vivo to the ASK-1 promoter in cycling cells, probably through a non-consensus E2F-binding site located 12 bp upstream of the transcription start site. Mutation of this site completely abolishes the ASK-1 promoter response to E2Fs as well as the E2F1 binding in electrophoretic mobility-shift experiments. Our results indicate that E2Fs modulate the expression of ASK-1 and suggest that some of the cellular functions of ASK-1 may be under the control of E2F transcription factors. Moreover, the up-regulation of ASK-1 may also favour the p53-independent E2F1 apoptotic activity.

Extracellular signal-regulated kinase (ERK) 5 is necessary and sufficient to specify cortical neuronal fate

Multipotent cortical progenitor cells differentiate into neurons and glial cells during development; however, mechanisms governing the specification of progenitors to a neuronal fate are not well understood. Although both extrinsic and intrinsic factors regulate this process, little is known about kinase signaling mechanisms that direct neuronal fate. Here, we report that extracellular signal-regulated kinase (ERK) 5 is expressed and active in proliferating cortical progenitors. Lentiviral gene delivery of a dominant negative ERK5 or dominant negative MAP kinase kinase 5 reduced the number of neurons generated from rat cortical progenitor cells in culture, whereas constitutive activation of ERK5 increased the production of neurons. Furthermore, when cortical progenitor cells were treated with ciliary neurotrophic factor, which induces precocious glial differentiation, ERK5 activation still promoted neuronal fate while suppressing glial differentiation. Our data also indicate that ERK5 does not directly regulate proliferation or apoptosis of cultured cortical progenitors. We conclude that ERK5 is necessary and sufficient to stimulate the generation of neurons from cortical progenitors. These results suggest a previously uncharacterized function for ERK5 signaling during brain development and raise the interesting possibility that extrinsic factors may instruct cortical progenitors to become neurons by activating the ERK5 pathway.

E2F1 stability is regulated by a novel-PKC/p38beta MAP kinase signaling pathway during keratinocyte differentiation

E2F transcription factors regulate proliferation, differentiation, DNA repair and apoptosis. Tight E2F regulation is crucial for epidermal formation and regeneration. However, virtually nothing is known about the molecular events modulating E2F during epidermal keratinocyte differentiation. Elucidation of these events is essential to understand epidermal morphogenesis, transformation and repair. Here we show that, in differentiating keratinocytes, Ca(2+)-induced protein kinase C (PKC) activation downregulates E2F1 protein levels. Further, we have identified PKC delta and eta as those isoforms specifically involved in induction of E2F1 proteasomal degradation. We also demonstrate that E2F1 downregulation by novel PKC isozymes requires activation of p38beta mitogen-activated protein kinase (MAPK). This is the first example of regulation in the E2F transcription factor family by activation of PKC and MAPK in the context of biologically significant differentiation stimuli in epithelia.

The role of p38 MAP kinase and c-Jun N-terminal protein kinase signaling in the differentiation and apoptosis of immortalized neural stem cells

The two distinct members of the mitogen-activated protein (MAP) kinase family c-Jun N-terminal protein kinase (JNK) and p38 MAP kinase, play an important role in central nervous system (CNS) development and differentiation. However, their role and functions are not completely understood in CNS. To facilitate in vitro study, we have established an immortal stem cell line using SV40 from fetal rat embryonic day 17. In these cells, MAP kinase inhibitors (SP600125, SB202190, and PD98059) were treated for 1, 24, 48, and 72 h to examine the roles of protein kinases. Early inhibition of JNK did not alter phenotypic or morphological changes of immortalized cells, however overexpression of Bax and decrease of phosphorylated AKT was observed. The prolonged inhibition of JNK induced polyploidization of immortalized cells, and resulted in differentiation and inhibition of cell proliferation. Moreover, JNK and p38 MAP kinase but not ERK1/2 was activated, and p21, p53, and Bax were overexpressed by prolonged inhibition of JNK. These results indicate that JNK and p38 MAP kinase could play dual roles on cell survival and apoptosis. Furthermore, this established cell line could facilitate study of the role of JNK and p38 MAP kinase on CNS development or differentiation/apoptosis.

Neural stem cell differentiation is dependent upon endogenous caspase 3 activity

Caspase proteases have become the focal point for the development and application of anti-apoptotic therapies in a variety of central nervous system diseases. However, this approach is based on the premise that caspase function is limited to invoking cell death signals. Here, we show that caspase-3 activity is elevated in nonapoptotic differentiating neuronal cell populations. Moreover, peptide inhibition of protease activity effectively inhibits the differentiation process in a cultured neurosphere model. These results implicate caspase-3 activation as a conserved feature of neuronal differentiation and suggest that targeted inhibition of this protease in neural cell populations may have unintended consequences.